In the cold drawing operation, the phosphate film is further covered by a lubricant such as metallic soap, and the phosphate film may carry the metallic soap forming a lubricative layer. In the cold drawing, a diameter of the steel wires become decreased by passing through several dies, and it is preferred that the phosphate film may keep its excellent performance until the steel wires passes through the last die.
The phosphate film is often formed by dipping the steel wires into a bath where a solution for making the phosphate film are contained and no electrolytic deviced are provided. In this case, a productivity of the processing bath is able to be enhanced by increasing the travelling speed of the steel wires if a prompt formation of the phosphate film become possible.
Accordingly, a method being possible to form the phosphate film promptly is preferable. In order to form the phosphate film promptly, a liquid containing chemicals of forming phosphate film in high concentration has been used. However, in this case, a great deal of sludge is produced in the liquid, and it must be frequently removed in order to obtain a phosphate film of excellent quality.
In the process of forming the phosphate film on the steel wires, the steel wires must be descaled previously by dipping it into the hydrochloric or sulfuric acid solution. And steel wires being removed its oxide film by this descalling process are dipped into the forming solution of phosphate film.
The present inventors had found a electrolytic process where phosphate film can be formed promptly by using the steel wires as electrolytic cathode in a solution of not containing a complexing agent, and filed it in JP4-36498A.
According to the electrolytic process of JP4-35498A, a productivity of the phosphate film formation had been much enhanced, however, it does not improved the quality of the phosphate film, and the quality of the phosphate film by this process had almost been in the same level as those produced by conventional process.
JP6-322592A disclosed a formation of phosphate film on the steel wires by electrolytic process. However, this is a process of using the steel wires as anode. Also, this is a process of using the pulse electrolytic current in the electrolytic operation.